4B5B
Encyclopedia
In telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

, 4B5B is a form of data communications Block Coding. 4B5B maps groups of four bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

s onto groups of 5 bits, with a minimum density of 1 bits in the output. When NRZI-encoded, the 1 bits provide necessary clock
Clock signal
In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...

 transitions for the receiver. For example, a run of 4 bits such as 0000 contains no transitions and that causes clocking problems for the receiver. 4B/5B solves this problem by assigning each block of 4 consecutive bits an equivalent word of 5 bits. These 5 bit words are pre-determined in a dictionary and they are chosen to ensure that there will be at least two transitions per block of bits.

A collateral effect of the code is that more bits are needed to send the same information than with 4 bits. An alternate to using 4B5B coding is to use a scrambler.
Depending on the standard or specification of interest, there may be several 4b5b characters left unused. The presence of any of the "unused" characters in the data stream can be used as an indication that there is a fault somewhere in the link. Therefore, the unused characters can actually be used to detect errors in the data stream.

4B5B was popularized by Fiber distributed data interface
Fiber distributed data interface
Fiber Distributed Data Interface provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to . Although FDDI logical topology is a ring-based token network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its...

 (FDDI) in the mid-1980s, and was later adopted by
  • 100BASE-TX standard
    Standardization
    Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....

     defined by IEEE 802.3u in 1995.
  • AES10-2003 MADI http://www.aes.org/publications/standards/ (Multichannel Audio Digital Interface)


The name "4B5B" is generally taken to mean the FDDI version. Other 4-to-5-bit codes have been used for magnetic recording are known as group code recording
Group Code Recording
In computer science, group code recording refers to several distinct but related encoding methods for magnetic media. The first, used in 6250 cpi magnetic tape, is an error-correcting code combined with a run length limited encoding scheme...

 (GCR).

On optical fiber, the 4B5B output is NRZI-encoded. FDDI over copper (CDDI) uses MLT-3 encoding
MLT-3 encoding
MLT-3 encoding is a line code that uses three voltage levels...

 instead, as does 100BASE-TX.

Encoding table

Name 4b 5b Description
0 0000 11110 hex data 0
1 0001 01001 hex data 1
2 0010 10100 hex data 2
3 0011 10101 hex data 3
4 0100 01010 hex data 4
5 0101 01011 hex data 5
6 0110 01110 hex data 6
7 0111 01111 hex data 7
8 1000 10010 hex data 8
9 1001 10011 hex data 9
A 1010 10110 hex data A
B 1011 10111 hex data B
C 1100 11010 hex data C
D 1101 11011 hex data D
E 1110 11100 hex data E
F 1111 11101 hex data F
Q -NONE- 00000 Quiet (signal lost)
I -NONE- 11111 Idle
J -NONE- 11000 Start #1
K -NONE- 10001 Start #2
T -NONE- 01101 End
R -NONE- 00111 Reset
S -NONE- 11001 Set
H -NONE- 00100 Halt


Note that normal data symbols begin with at most one 0 bit and end with at most two, so there can be at most three 0 bits in a row. Control symbols are used in combinations that also preserve this rule. Thus, 4B5B encoding is a (0,3) RLL code
Run Length Limited
Run length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. This is used in both telecommunication and storage systems which move a medium past a fixed head. Specifically, RLL bounds the length of stretches ...

.

FDDI and 100BASE-TX begin frames with a JK pair. FDDI ends frames with a TT pair, while 100BASE-TX uses a TR pair.

The following character sets are sometimes referred to as command characters

Control Character 5b symbols Purpose
JK 11000 10001 Sync, Start delimiter
II 11111 11111 Not Used
TT 01101 01101 FDDI end delimiter
TS 01101 11001 Not Used
IH 11111 00100 SAL
TR 01101 00111 100BASE-TX end delimiter
SR 11001 00111 Not Used
SS 11001 11001 Not Used
HH 00100 00100 HDLC0
HI 00100 11111 HDLC1
HQ 00100 00000 HDLC2
RR 00111 00111 HDLC3
RS 00111 11001 HDLC4
QH 00000 00100 HDLC5
QI 00000 11111 HDLC6
QQ 00000 00000 HDLC7


(HDLC = High-Level Data Link Control
High-Level Data Link Control
High-Level Data Link Control is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization...

)

External links

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